The art of steel making is very well developed. In general, an electric arc furnace (EAF) is used to make steel by application of an electric arc to melt one or more of scrap metal and/or other alternative iron bearing feed stocks and alloys that are placed within the furnace. Other methods to make steel include enhanced versions of EAFs that make steel by melting DRI (direct reduced iron) combined with the hot metal from a blast furnace. To enhance the steel making process, additional chemical energy is provided to the furnace by auxiliary means. The most common forms of auxiliary means comprise burners, injectors, and jets using fuel and an oxidizing gas to produce combustion products with a high heat content to assist the arc.
Further processes include multiple movable or permanently fixed burners utilizing hydrocarbon fuel such as, for example, natural gas or oil, at least one movable oxygen lance for injection of a stream of oxygen toward the molten bath for refining purposes and a movable means for injecting solid carbonaceous fuel for combustion and slag foaming purposes.
In the general process of EAFs, scrap metal, or charges, are dumped into the furnace through an opening. Typically these charges further include carbon particulate and other slag forming materials. Other known processes comprise using a ladle for hot or heated metal from a blast furnace and inserting it into the EAF furnace, such as by injection of the DRI by a lance.
There are numerous phases of charge processing in an EAF furnace and/or an EAF-like furnace. In the melting phase, the electric arc and burners melt the charge burden into a molten pool of metal (melted metal), called an iron carbon melt, which accumulates at the bottom or hearth of the furnace. Most commonly, after melting the charge, an electric arc furnace proceeds to a refining and/or decarburization phase. In this phase, the metal melt continues to be heated by the arc until slag forming materials combine with impurities in the iron carbon melt and rise to the surface as slag. When the iron carbon melt reaches a critical temperature which allows a carbon boil, the charged carbon in the melt combines with any oxygen present in the bath to form carbon monoxide bubbles which rise to the surface of the bath, forming foaming slag. The foaming slag acts as an insulator throughout the furnace.
When an electric arc furnace operates without burners, the charged scrap or charge rapidly melts at the hot spots at regions of highest electric current density, but often remains un-melted at the cold spots (those spots of lowest electric current density). This creates harsh conditions for the portion of the furnace wall and refractory lining located at the hot spots due to excessive exposure to heat from the arc during the latter portions of the melt down cycle. Scrap located in the cold spot regions receives heat from the arc at a reduced rate during the melt down cycle, thereby creating the cold spots. To melt charge scrap in the cold spots, the heat is applied for a longer period of total time, thereby also resulting in applying heat to the hot spots for longer than necessary. This asymmetrical heat distribution from the arc and non-uniform wear of the furnace walls are typical for both alternating current and direct current arc furnaces operating without burners.
The cold spots are typically formed in areas further away from the furnace arc as scrap located in these areas receives electrical energy at a reduced rate per ton of scrap. The electrical energy is the weakest in line with the bisect of the angle between the electrodes forming cold spots. Another typical example of a cold spot is the tapping spout, due to its location away from the arc. Still another cold spot occurs at the slag door due to excessive heat losses to ambient air which infiltrates through this area. An even further common source for cold spots in furnaces occurs at the places where additional materials are injected, such as slag forming material, direct reduced iron, lime, etc., (which is inserted through a slag door or through an opening in the furnace side wall) due to the heat consumption of these materials as they melt down.
Prior art solutions to these challenges have been to incorporate further burners around the furnace in order to apply additional sources of heat to the cold spots. Electric arc furnaces equipped with burners located at cold spots have improved uniformity of scrap melting and have reduced build-ups of materials at the cold spots. When auxiliary heat sources such as burners are placed in the electric arc furnace, their location is chosen to avoid further overheating of hot spots that result from the rapid melting of scrap located between the electrode and the furnace shell. More specifically, the burners are located as far away from hot spots as is practically possible and the burner flame outlet opening direction is chosen so that flame penetration occurs predominantly into the scrap pile located at the cold spots and not to already heated portions of the furnace.
Further heating and processing is realized by a decarburization process wherein, in typical embodiments of the prior art utilizing advanced or more modern EAF techniques, a high velocity, usually supersonic, flow(s) of oxygen is blown into the metal bath with either lances or burner/lances to decarburize the bath by oxidation of the carbon contained in the bath, forming CO and/or CO2 when combined with the available or excess carbon in the bath. The burner(s)/lance(s) act more uniformly to melt the charge and lessen, or prevent, overheating and minimize the time required for the melt and time that the arc is operated.
By injecting the metal bath or liquid metal with oxygen, the dissolved carbon content of the bath can be reduced to a selected or reduced level. It is commonly regarded that if an iron carbon melt is under 2% carbon, the melt becomes steel. EAF steel making processes typically begin with burdens having less than 1% carbon. The carbon in the steel bath is continually reduced until it reaches the content desired for producing a specific grade of steel, such as, for example, and not by way of limitation, down to less than 0.1% for low carbon steels.
In an effort to decrease steel production times in electric arc furnaces, various apparatus and methods have been developed to alter the means of delivering further energy to the furnace. Such improvements include, but are not limited to, conventional burners mounted on the water-cooled side walls (panels or furnaces), conventional lances, conventional burners, and/or the like.
Typically, oxygen injection for the decarburization must wait until the melting phase of the process is substantially complete before starting high velocity injection of oxygen. This is since the burners cannot effectively deliver high velocity oxygen before then because unmelted charge may exist between the burners/lances and the liquid metal or metal melt. The oxygen flow would be deflected, potentially causing severe damage to the furnace and burner/injector panel.
This fact is further aggravated by the generally spherical shape of most EAF furnace structures. Melting of the metal typically occurs in the middle, lower portion of the melt and expands to fill the sides. Early in the melting phase a high velocity oxygen stream has less effect and/or ability to penetrate a not fully melted charge (metal) to decarburize the metal melt.
The same philosophy that is used in selecting the location of additional burners is used to select the location of other additional auxiliary heat sources including oxygen injection lances for use in decarburization. When additional lances are located at the cold spot(s), the exothermic energy of melt refining can be used more effectively to melt the scrap without overheating the hot spots.
Additional injection of oxygen for melt decarburization can be accomplished by any number of means. Common types of apparatus and processes include one or more movable devices, such as submerged, consumable oxygen pipes and/or by one or more water-cooled non-submerged oxygen lances. Often during operation of a water-cooled lance, the lance is first introduced into the furnace, then gradually moved to the position (sometimes in a fixed position) in which the lance discharge opening or openings for the introduction of oxygen are located, preferably approximately 150-300 mm or more above the bath. The discharge velocity of the oxygen stream from the lance is to be chosen to allow the stream of oxygen introduced by the lance located in the working position to penetrate the slag and to react with the iron-carbon melt without excessive molten metal splashing on the furnace walls and electrode(s). However, inadvertent metal splashing does occur and is a common cause of apparatus failure.
Combined injection of carbon and oxygen via various apparatus, including dedicated lances in and around the furnace wall has become a common practice for adding extra heat to the process. Typically, the supply of carbon flow for injection is obtained from a carbonaceous material dispenser, such as a compressed gaseous carrier comprising compressed air, natural gas, nitrogen, and/or the like.
The use of the burners together with carbon and oxygen lances has allowed electric steelmakers to substantially reduce electrical energy consumption and to increase furnace production rate due to the additional heat input generated by the oxidation of carbon, and by significant increases in electric arc thermal efficiency achieved by the formation of a foamy slag layer that insulates the electric arc from heat losses. The foamy slag also stabilizes the electric arc and therefore allows for a higher electrical power input rate. The foamy slag layer is created by CO bubbles which are formed by the oxidation of injected carbon to CO. The increased flow of injected carbon creates increased localized CO generation. Accordingly, most EAF furnace units also comprise a post production means for removing or reducing CO levels in the off gas. Mixing of the CO with oxygen inside of the electric arc furnace is desirable but very difficult to arrange without excessive oxidation of the slag and electrodes. Accordingly, the art field has developed post-production means for treating the high CO content of the off gas.
Most modern electric arc furnaces are equipped with all or some of the above-mentioned means for auxiliary heat input and or metal melting. Along with improvements in the design and operation of metal melting furnaces have come improvements in burner panel design. For example, various burner panel configurations are disclosed in U.S. Pat. No. 4,703,336; U.S. Pat. No. 5,444,733; U.S. Pat. No. 6,212,218; U.S. Pat. No. 6,372,010; U.S. Pat. No. 5,166,950; U.S. Pat. No. 5,471,495; U.S. Pat. No. 6,289,035; U.S. Pat. No. 6,614,831; U.S. Pat. No. 5,373,530; U.S. Pat. No. 5,802,097; U.S. Pat. No. 6,999,495; and U.S. Pat. No. 6,342,086. Such prior art patents have proven to be beneficial. For example, U.S. Pat. No. 6,999,495 has found wide applicability for increasing spatial energy coverage in a furnace. Likewise, U.S. Pat. No. 6,614,831 has found applicability in extending the reach of various tools, such as a burner or a lance, into the interior of a furnace. However, there is still a need for further improved apparatus and methods for the melting of metals which are even more efficient than the prior art apparatus and methods and which result in a decrease in burner/injector panel failure.
One of the causes of burner/injector panel failure is “flashback”, “blowback”, “rebound”, and/or “jet reflection”. These terms commonly refer to a condition resulting from the jet (oxygen lance or burner jet) being reflected back to the panel regardless of whether the reflection is caused from the steel bath or melting metals (scrap materials inside the furnace that are not yet melted). The use of the term “flashback” throughout this specification shall mean and refer to all of the aforementioned terms unless specifically stated otherwise. Prior art solutions to various challenges associated with flashback have been dealt with by shielding the burner jet and/or lance. However, shielding often results in increasing the distance from the burner or lance to the steel bath or melting metals. Accordingly, there is a need for apparatus and methods in which the distance from a burner jet nozzle or lance nozzle to the molten metal is minimized while at the same time providing enhanced shielding and/or protection for a burner jet and/or burner jet nozzle.
Another problem with the prior art is that it's application has been mainly limited to larger furnaces (those have a capacity greater than 40 tons). Therefore, there is a need to have a panel that will work not only in large capacity furnaces but also smaller furnaces.
An additional problem with the panels of the prior art is that the cooling circuits in the panels have been made (formed in the panels) using sand casting processes. When using sand casting, it is necessary to drill holes into the cooling circuits of the panel in order to remove the sand. Plates or plugs then have to be welded over the holes to preserve the integrity of the cooling circuit within the panel (to make certain that the water does not seep out of the cooling circuit and into the furnace). In such cases it is even more important to protect the panel from splash back since repeated exposure of the plates to molten metal can result in seepage of water from the cooling system into the furnace. The presence of water creates an unacceptable hazard. As a result, the panel will have to be removed and in many instances replaced. Accordingly, there is also a need to have a panel that can overcome this problem.